Method for adapting filter cut-off frequencies for the transmission of discrete multitone symbols

ABSTRACT

The invention creates a method for adapting filter cut-off frequencies for the transmission of discrete multitone symbols, where a transmit symbol datastream consisting of discrete multitone symbols is applied to an interpolation device, the transmit symbol datastream is interpolated with a symbol rate in the interpolation device, an interpolated symbol datastream is filtered in a first low-pass filtering device in accordance with a first filter cut-off frequency, which can be predetermined by a first filter cut-off frequency determining device, a digital symbol datastream obtained after a digital-analog conversion, transmission and analog-digital conversion, is filtered at the receiver end in a second low-pass filtering device in accordance with a second filter cut-off frequency, which can be predetermined by a second filter cut-off frequency determining device, in order to provide an equalized symbol datastream, the equalized symbol datastream is decimated in a decimation device and the decimated received symbol datastream consisting of discrete multitone symbols is provided to a multitone receiver device.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/175,332 filed Jun. 19, 2002, which claims priority to Germanapplication number DE 10129317.8filed Jun. 19, 2001.

FIELD

The present invention relates to a method for transmitting an analogdatastream and, in particular, relates to a method for adapting filtercut-off frequencies for the transmission of discrete multitone symbols,in which method transient effects are reduced.

BACKGROUND

Multicarrier systems consisting of a large number of orthogonalquadrature-amplitude modulated (QAM) carriers are normally provided fortransmitting analog datastreams. Such transmission systems and methodsare described, for example, in “J. Bingham, Multicarrier modulation fordata transmission: an idea whose time has come, IEEE Commun. Mag., Vol.28, May 1990, pp. 5-14”. In discrete multitone modulation (DMT), a timedomain equalizer is normally used for restricting the length of thechannel to less than the length of one cyclic prefix (described indetail below). Usually, a multitone method (DMT—Discrete Multitone) isused for asymmetric datastream transmission via normal telephone lines,normal telephone lines usually being constructed as asymmetric digitalsubscriber lines (ADSL).

High-speed digital subscriber lines of the prior art are described, forexample, in the publication “High-speed digital subscriber lines, IEEEJournal Sel. Ar. In Comm., Vol. 9, No. 6, August 1991”. Among thetransmission methods with a high data rate, which are based on digitalsubscriber lines (DSL), a number of VDSL (Very High Data Rate DSL)arrangements are known and, for example, methods such as carrierlessamplitude/phase (CAP), discrete wavelet multitone (DWMT), single linecode (SLC) and discrete multitone (DMT) can be used for these. In theDMT method, the transmit signal is provided from multiple sinusoidal orcosinusoidal signals, where both the amplitude and the phase can bemodulated of each individual sinusoidal or cosinusoidal signal. Themultiple modulated signals thus obtained are provided asquadrature-amplitude modulated (QAM) signals.

FIG. 4 shows a conventional arrangement for transmitting discretemultitone (DMT) symbols from a multitone transmitter device 223 to amultitone receiver device 222. In the method of the prior art shown, adatastream consisting of multitone symbols is input by the multitonetransmitter device 223 into an interpolation device 214. The datastreaminterpolated by the interpolation device 214 is then supplied to a firstlow-pass filter 401 which has a fixed cut-off frequency and providesfiltering of the interpolated symbol datastream.

In a subsequent digital-analog converter 204, the filtered digitaldatastream is converted into an analog datastream and transmitted via atransmission channel 102. The transmitted analog datastream isreconverted into a digital datastream in an analog-digital converter 104and supplied to a second fixed low-pass filter 402 which has a secondfixed cut-off frequency.

Finally, the filtered digital datastream transmitted is supplied to adecimation device 107 in which the digital datastream is decimated. Thedecimated datastream is forwarded as a received symbol datastream to themultitone receiver device 222 in which further processing of thereceived symbol datastream is performed.

An essential disadvantage of data transmission according to the DMTmethod via lines, for example twisted copper wire lines, consists inthat long transient effects occur which limit a transmittable bandwidth.

Furthermore, it is unsuitable that fixed low-pass filters are used inorder to limit the bandwidth of the analog datastream to be transmittedand to limit out-of-band noise in analog-digital and digital-analogconverters which can be constructed, for example, as sigma-deltaconverters.

In particular, it is disadvantageous that, when low-pass filters areexcited with DMT signals, transient effects can occur which haveconsiderable spectral components above the intended transmission band ina frequency range.

A further disadvantage of conventional methods and circuit arrangementsfor transmitting analog datastreams which have multitone signalsconsists in that in the transmission signal band convolution productsoccur which cannot be eliminated by a multitone receiver device.

It is also unsuitable that these convolution products are contained asinterference signals in the transmission signal band as a result ofwhich the quality of transmission is impaired and the bandwidth islimited.

SUMMARY

One concept of the invention includes low-pass filtering of aninterpolated symbol datastream in a first low-pass filtering deviceaccording to a first filter cut-off frequency, which can be variablypredetermined by a first filter cut-off frequency determining device,and filtering a received digital symbol datastream in a second low-passfiltering device in accordance with a second filter cut-off frequencywhich is variably predeterminable by a second filter cut-off frequencydetermining device.

It is thus an advantage of the present invention that transient effectscan be reduced in a transmission of analog datastreams which are builtup from multi tone symbols.

It is also advantageous that low-pass filtering is provided variablybased on one design of a DMT transmission system.

The method according to the invention for adapting filter cut-offfrequencies for the transmission of discrete multitone symbolsessentially comprises the following steps:

-   a) applying a transmit symbol datastream consisting of discrete    multitone symbols, which is provided by a multitone transmitter    device, to an interpolation device to which a symbol rate is    applied;-   b) interpolating the symbol datastream with the symbol rate in the    interpolation device in order to provide an interpolated symbol    datastream;-   c) low-pass filtering of the interpolated symbol datastream in a    first low-pass filtering device according to a first filter cut-off    frequency, which can be variably or adaptively predetermined by a    first filter cut-off determining device, in order to provide a    filtered symbol datastream;-   d) converting the filtered symbol datastream into an analog    datastream in a digital-analog converter, in order to provide an    analog datastream via [sic] a transmission via a transmission    channel;-   e) transmitting the analog datastream via the transmission channel;-   f) converting the transmitted analog datastream into a digital    symbol datastream in an analog-digital converter;-   g) low-pass filtering the digital symbol datastream in a second    low-pass filtering device in accordance with a second filter cut-off    frequency, which can be variably or adaptively predetermined by a    second filter cut-off frequency determining device, in order to    provide an equalized symbol datastream;-   h) decimating the equalized symbol datastream in a decimation    device; in order to provide a decimated received symbol datastream    consisting of discrete multitone symbols; and-   i) delivering the received symbol datastream to a multitone receiver    device in which the received datastream is analyzed or processed    further.

According to a preferred further development of the present invention,the first filter cut-off frequency, which is predeterminable by thefirst filter cut-off frequency determining device, is variably adjustedduring low-pass filtering of the interpolated symbol datastream in thefirst low-pass filtering device.

According to a further preferred further development of the presentinvention, the first filter cut-off frequency, which is predeterminableby the first filter cut-off frequency determining device, is adaptivelyadjusted in accordance with the multitone symbol to be transmitted,during low-pass filtering of the interpolated symbol datastream in thefirst low-pass filtering device.

According to yet another preferred development of the present invention,the second filter cut-off frequency, which is predeterminable by thesecond filter cut-off frequency determining device, is variably adjustedin the second low-pass filtering device during low-pass filtering of thedigital symbol datastream which is obtained from the analog-digitalconverter.

According to yet another preferred development of the present invention,the second filter cut-off frequency, which is predeterminable by thesecond filter cut-off frequency determining device, is adaptivelyadjusted in the second low-pass filtering device during low-passfiltering of the digital symbol datastream which is obtained from theanalog-digital converter.

According to yet another preferred development of the present invention,the filtered symbol datastream is oversampled with a sampling rateduring a conversion of the filtered symbol datastream into the analogdatastream in the digital-analog converter.

According to yet another preferred development of the present invention,the transmitted analog datastream is oversampled with a sampling rateduring a conversion of the transmitted analog datastream into thedigital datastream in the analog-digital converter.

According to yet another preferred development of the present invention,the first filter cut-off frequency of the first low-pass filteringdevice is changed during the cyclic prefix by the first filter cut-offfrequency determining device.

According to yet another preferred development of the present invention,the second filter cut-off frequency of the second low-pass filteringdevice is changed during the cyclic prefix of a DMT symbol.

The circuit arrangement according to the invention for adapting filtercut-off frequencies for the transmission of discrete multitone symbolsalso exhibits the following:

-   a) a multitone transmitter device for providing a transmit symbol    datastream consisting of discrete multitone symbols;-   b) an interpolation device for interpolating the transmit symbol    datastream consisting of discrete multitone symbols, in order to    provide an interpolated symbol datastream;-   c) a first filter cut-off frequency determining device for providing    a first filter cut-off frequency which is variably or adaptively    predeterminable;-   d) a first low-pass filtering device for low-pass filtering of the    interpolated symbol datastream in accordance with the first filter    cut-off frequency predetermined by the first filter cut-off    frequency determining device, in order to provide a filtered symbol    datastream;-   e) a digital-analog converter for converting the filtered symbol    datastream into an analog datastream;-   f) a transmission channel for transmitting the analog datastream;-   g) an analog-digital converter for converting the transmitted analog    datastream into a digital symbol datastream, the analog-digital    converter operating at a predeterminable sampling rate;-   h) a second filter cut-off frequency determining device for    providing a second filter cut-off frequency for a second low-pass    filtering device;-   i) a second low-pass filtering device for low-pass filtering of the    digital symbol datastream in accordance with a second filter cut-off    frequency predetermined by the second filter cut-off frequency    determining device; in order to provide an equalized symbol    datastream;-   j) a decimation device for decimating the equalized symbol    datastream in order to provide a decimated received symbol    datastream consisting of discrete multitone symbols; and-   k) a multitone receiver device for further processing of the    received symbol datastream.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are explained in greater detailin the description following and represented in the drawings, in which:

FIG. 1 shows a circuit arrangement for transmitting data by means of themultitone method from a multitone transmitter device to a multitonereceiver device according to an exemplary embodiment of the presentinvention;

FIG. 2 a shows a block diagram of a DMT transmission link withdatastream transmitter, transmission channel and datastream receiver;

FIG. 2 b shows the structure of a discrete multitone symbol preceded bya cyclic prefix;

FIG. 3 shows the transmission arrangement for transmitting the analogdatastream, illustrated in FIG. 2 a, in greater detail as a completelink; and

FIG. 4 shows a circuit arrangement of a conventional multitonetransmission system with a first fixed low-pass filter and a secondfixed low-pass filter.

DETAILED DESCRIPTION

In the figures, identical reference symbols designate identical orfunctionally equal components or steps.

FIG. 2 a shows a basic block diagram of an arrangement for transmittingan analog datastream according to the DMT method, the datastreamtransmitter 210, the transmission channel 102 and the datastreamreceiver 211 being illustrated.

Datastream transmitter 210 and datastream receiver 211 consist ofseparately identifiable blocks which will be briefly described in thetext which follows. A data input device 201 is used for inputting datato be transmitted, the input data being forwarded to a coding device202. In the coding device 202, the datastream is decoded in accordancewith a conventional method and supplied to a retransformation device203.

The retransformation device 203 provides a transformation of datapresent in the frequency domain into data present in the time domain.The retransformation device 203 can be provided, for example, by adevice in which an inverse fast Fourier transformation (IFFT) isperformed.

It should be pointed out that the transformation from the frequencydomain into the time domain performed in the retransformation device 203represents a transformation which is inverse to the transformationperformed by the transformation device 110.

Finally, the digital datastream output by the retransformation device203 is converted into an analog datastream by means of a digital-analogconverter 204. The analog datastream, which is now present in the timedomain, is supplied to a transmission channel 102 which provides thedata transmission described above, and for the transmission, there canbe band-pass filtering, high-pass filtering and/or low-pass filteringand an application of noise to the analog datastream 101. The analogdatastream 101 is furthermore supplied to the analog-digital converter104 arranged in the datastream receiver 211, which converts the receivedanalog datastream 101 into a digital datastream 103, the converteddigital datastream 103 being supplied to the transformation device 110.

After a transformation, which is the inverse to that in theretransformation device 203, from the frequency domain into the timedomain, the transformed datastream, after passing through a correctiondevice (not shown) and a determining device (not shown), is decoded inthe decoding device 117. The decoded datastream is finally output viathe data output device 119.

FIG. 2 b shows an arrangement of a discrete multitone symbol, the analogdatastream to be transmitted being provided as a sequence of multitonesymbols. Before the data transformed in the transformation device 203are forwarded to the digital-analog converter 204, the last M samples ofa multitone symbol are again appended to the start of the block whichdefines a cyclic prefix and where the following applies:

M<N

This makes it possible to simulate a periodic signal for a datastreamreceiver if the transient effect caused by the transmission channel hasdecayed after M samples, i.e. there is no inter-symbol interference(ISI).

As shown in FIG. 2 b, the original multitone symbol has a length of Nsamples, for example N=64 whereas, for example, the last four values areplaced at the start of the DMT symbol 205 as a cyclic prefix 212, where:

M=4.

The total length of a multitone symbol 208, together with the end of DMTsymbol values 213 appended to the start of the symbol 205, is then M+Nfrom the start of prefix 207 to the end of DMT symbol 206.

It should be pointed out that the number of end of DMT symbol values 213cyclically appended to the start of symbol 205 must be kept as small aspossible, i.e. M<<N in order to obtain the least possible reduction intransmission capacity and quality.

In another example, a multitone symbol 208 consists of 256 complexnumbers which means that 512 time samples (real and imaginary component)must be transmitted as a periodic signal. In this example, if a total of32 end of DMT symbol values 213 are copied to the start of the symbol ascyclic prefix 212, to be total length of the time sample to betransmitted is calculated to be 544; which results in a sampling periodT.sub.A of 544.times.10.sup.−6/2.208 s or 0.25 ms, at a maximum tonefrequency of a DMT signal of 2.208 MHz, the symbol transmissionfrequency being calculated from f.sub.DMT=1/T.sub.A.apprxeq.4 kHz.

FIG. 3 shows a method for transmitting an analog datastream and acircuit arrangement in a more detailed representation.

The datastream supplied to the data input device 201 is combined intoblocks and a certain number of bits to be transmitted is allocated to acomplex number depending on scaling. In the coding device 202, finally,coding takes place in accordance with the selected scaling, the codeddatastream finally being supplied to the retransformation device 203.

A multitone signal 303 provided by the retransformation device 203finally forms a digital transmitter datastream which has beentransformed from the frequency domain into the time domain. Themultitone signal 303 formed as a digital datastream is finally convertedinto an analog datastream in the digital-analog converter 204 andsupplied to a line driver device 304.

The line driver device 304 amplifies or drives, respectively, the analogdatastream 101 to be transmitted into a transmission channel 102, thechannel transfer function of which is known in principle or can bemeasured. In the transmission channel, noise is also superimposed on theanalog datastream which is shown by a superposition device 121 in FIG.3. The superposition device 121 is supplied with the analog datastreamtransmitted from the transmission channel and with a noise signal 122 sothat, finally, an analog datastream 101 is obtained on which noise issuperimposed.

The analog datastream 101 is supplied to a preprocessing device 301which, according to the invention, contains the analog-digital converter104 shown in FIG. 1, the second low-pass filtering device 105 and thedecimation device 107 in the order shown in FIG. 1.

A preprocessed digital datastream 302 output by the preprocessing device301 is finally supplied to the circuit units of the datastream receiver.The transmitted analog datastream 101′, which has been transmitted via atransmission channel 102 which can be noisy, is supplied to ananalog-digital converter 104 which samples the analog datastream 101with a sampling rate 108, an equidistant sampling of the transmittedanalog datastream 101′ being provided. The analog-digital converter 104converts the transmitted analog datastream 101′ into a digital symboldatastream 103 which, in turn, is supplied to a second low-passfiltering device 105.

The transformation device 110 provides a transformation of the decimatedequalized digital datastream 109 into transformation signals 111 a-111n, where n represents the maximum number, 256 in the present example, ofthe cosinusoidal or sinusoidal signals defined in amplitude and phase.It should be pointed out that the transformation device 110 performs adigital transformation of a signal which is digitally present in thetime domain into a signal which is digitally present in the frequencydomain.

The transformation signals 111 a-111 n correspond, for example, tocomplex numbers for each of the multitones, evaluation being provided inamplitude and phase or, respectively, as a real component and imaginarycomponent. Furthermore, the complex numbers can be provided asamplitudes of cosinusoidal (real component) and sinusoidal oscillations(imaginary component) to be sent out within a block, the frequenciesbeing provided equidistantly distributed in accordance with the equationspecified above, the data to be transmitted being combined in blocks.

It should be pointed out that more or fewer than 256 different tones canbe transmitted as cosinusoidal or sinusoidal signals which are definedand can be modulated in amplitude and phase, resulting in acorrespondingly different number of transformation signals 111 a-111 n.The first transformation signal is here designated as 111 a and the lasttransformation signal as 111 n. The transformation device 110 preferablycarries out a fast Fourier transform (FFT) in order to provide a fasttransformation from the time domain into the frequency domain.

In a correction device 112, the transformation signals 111 a-111 n areweighted with a known correction function which is input to thecorrection device 112. This correction function input into thecorrection device 112 is preferably but not exclusively an inverse ofthe channel transfer function of the transmission channel. This makes itpossible to compensate for influences of the transmission channel withrespect to frequency response, phase etc. so that correctedtransformation signals 113 a-113 n are obtained at the output of thecorrection device 112. The corrected transformation signals 113 a-113 nare then supplied to a determining device 116 in which at least onemagnitude signal 114 and at least one phase signal 115 or, respectively,a real component and an imaginary component, of a correctedtransformation signal is determined.

The magnitude signals 114 and phase signals 115, determined in thedetermining device, are then decoded by supplying the magnitude signals114 and the phase signals 115 to a decoding device 117.

In the decoding device 117, decoding according to a coding of thedatastream performed in the datastream transmitter 225 (described below)is provided. The decoding device 117 thus outputs a decoded datastream118 which is finally supplied to a data output device 119 and can beoutput from there and processed further.

FIG. 1 shows a circuit arrangement for transmitting an analog datastreamin which the filter cut-off frequencies 219 and 221, respectively, areadapted during a transmission of discrete multitone symbols 208 as aresult of which transient effects are reduced.

A multitone transmitter device 223 supplies a transmit symbol datastream209 to an interpolation device 214. The interpolation device 214operates with a symbol rate 120 generated by means of a symbol rategenerating device 211, the transmit symbol datastream 209 beinginterpolated with the symbol rate 120 in order to provide aninterpolated symbol datastream 215 at the output of the interpolationdevice 214. The interpolated symbol datastream 215 is supplied to afirst low-pass filtering device 216 where a first filter cut-offfrequency 219 is input to the first low-pass filtering device 216 by afirst filter cut-off frequency determining device 218.

In the exemplary embodiment of the present invention shown, the firstfilter cut-off frequency 219 of the first low-pass filtering device 216can be reprogrammed during a cyclic prefix 212 of a DMT symbol 208, i.e.from higher first filter cut-off frequencies to an actual band limit inorder to reduce transient effects in accordance with the invention.

Furthermore, the first low-pass filtering device 216 can be provided asan adaptive filtering device. A transient effect can be reduced furtherby corresponding oversampling. The filtered symbol datastream 217 outputby the first low-pass filtering device 216 is supplied to adigital-analog converter 204 which operates at a first sampling rate210. The symbol datastream, filtered and converted to form an analogdatastream by the digital-analog converter 204, is supplied to atransmission channel 102 via which the analog datastream 101 istransmitted. The transmitted analog datastream 101′ provided at theoutput of the transmission channel 102 is supplied to an analog-digitalconverter 104 which operates at a second sampling rate 108. In theanalog-digital converter 104, the transmitted analog datastream 101′ isconverted into a digital symbol datastream which is then supplied to asecond low-pass filtering device 105.

According to the invention, a second filter cut-off frequency of thesecond low-pass filtering device can be variably adjusted. Inparticular, the second filter cut-off frequency 221 is provided via asecond filter cut-off frequency determining device 220 and supplied tothe second low-pass filtering device 105. According to the invention,the second filter cut-off frequency 221 of the second low-pass filteringdevice 105 can be reprogrammed during the cyclic prefix 212, i.e. variedfrom higher second filter cut-off frequencies 221 to the actual bandlimit, in order to reduce transient effects.

Furthermore, it is possible that the second low-pass filtering device105 is constructed as an adaptive filter as a result of which adaptivefiltering is achieved. The transient effect can be minimized further bycorresponding oversampling in the decimation path. An equalized symboldatastream 106 output by the second low-pass filtering device 105 issupplied to a decimation device 107 which generates from the equalizedsymbol datastream 106 a decimated received symbol datastream 109consisting of discrete multitone symbols 208.

The received symbol datastream 109 generated is supplied to a multitonereceiver device 222 in which an analysis and further processing of thereceived symbol datastream 109 is performed.

The low-pass filtering devices 216 and 105, respectively, shown in FIG.1, can be constructed as adaptive filtering devices in such a mannerthat filtering beginning from a high filter cut-off frequency toward alower filter cut-off frequency is provided.

The first and second low-pass filtering devices 216 and 105 respectivelyare advantageously designed in such a manner that the low-pass filteringdevices settle rapidly. The first low-pass filtering device 216 can alsobe constructed as a first filtering device reprogrammed in the firstfilter cut-off frequency 219 whereas the second low-pass filteringdevice 105 can be constructed as a second filtering devicereprogrammable in the second filter cut-off frequency 221.

Reference is made to the introduction to the description with respect tothe conventional circuit arrangement for transmitting discrete multitonesymbols, shown in FIG. 4.

Although the present invention has been described above by means ofpreferred exemplary embodiments, it is not restricted to these but canbe modified in various ways.

1. A method for adapting filter cut-off frequencies for the transmissionof discrete multitone symbols, comprising the following steps: a)applying a transmit symbol datastream consisting of discrete multitonesymbols, which is provided by a multitone transmitter device, to aninterpolation device; b) interpolating the transmit symbol datastreamwith a symbol rate in the interpolation device to provide aninterpolated symbol datastream; c) low-pass filtering of theinterpolated symbol datastream in a first low-pass filtering deviceaccording to a first filter cut-off frequency, which is predetermined bya first filter cut-off determining device, to provide a filtered symboldatastream; d) converting the filtered symbol datastream into an analogdatastream in a digital-analog converter; e) transmitting the analogdatastream via a transmission channel; f) converting the transmittedanalog datastream into a digital symbol datastream in an analog-digitalconverter; g) low-pass filtering the digital symbol datastream in asecond low-pass filtering device in accordance with a second filtercut-off frequency, which is predetermined by a second filter cut-offfrequency determining device, to provide an equalized symbol datastream;h) decimating the equalized symbol datastream in a decimation device toprovide a decimated received symbol datastream having discrete multitonesymbols; and i) delivering the received symbol datastream to a multitonereceiver device.
 2. The method as claimed in claim 1, wherein the firstfilter cut-off frequency, which is predetermined by the first filtercut-off frequency determining device, is variably adjusted duringlow-pass filtering of the interpolated symbol datastream in the firstlow-pass filtering device.
 3. The method as claimed in claim 1, whereinthe first filter cut-off frequency, which is predetermined by the firstfilter cut-off frequency determining device, is adaptively adjusted inaccordance with the multitone symbol to be transmitted, during low-passfiltering of the interpolated symbol datastream in the first low-passfiltering device.
 4. The method as claimed in claim 1, wherein thesecond filter cut-off frequency, which is predetermined by the secondfilter cut-off frequency determining device, is variably adjusted in thesecond low-pass filtering device during low-pass filtering of thedigital symbol datastream.
 5. The method as claimed in claim 1, whereinthe second filter cut-off frequency, which is predetermined by thesecond filter cut-off frequency determining device, is adaptivelyadjusted in the second low-pass filtering device during low-passfiltering of the digital symbol datastream.
 6. The method as claimed inclaim 1, wherein the filtered symbol datastream is oversampled with asampling rate during a conversion of the filtered symbol datastream intothe analog datastream in the digital-analog converter.
 7. The method asclaimed in claim 1, wherein the transmitted analog datastream isoversampled with a sampling rate during conversion of the transmittedanalog datastream to the digital symbol datastream in the analog-digitalconverter.
 8. The method as claimed in claim 1, wherein the first filtercut-off frequency of the first low-pass filtering device is changedduring a cyclic prefix of a DMT symbol.
 9. The method as claimed in oneof claim 1, wherein the second filter cut-off frequency of the secondlow-pass filtering device is changed during the cyclic prefix.
 10. Acircuit arrangement for adapting filter cut-off frequencies duringtransmission of discrete multitone symbols, in which transient effectsare reduced, comprising the following: a) a multitone transmitter devicefor providing a transmit symbol datastream having discrete multitonesymbols; b) an interpolation device for interpolating the transmitsymbol datastream having discrete multitone symbols, to provide aninterpolated symbol datastream; c) a first filter cut-off frequencydetermining device for providing a first filter cut-off frequency; d) afirst low-pass filtering device for low-pass filtering of theinterpolated symbol datastream in accordance with the first filtercut-off frequency predetermined by the first filter cut-off frequencydetermining device, to provide a filtered symbol datastream; e) adigital-analog converter for converting the filtered symbol datastreamto an analog datastream; f) a transmission channel for transmitting theanalog datastream; g) an analog-digital converter for converting thetransmitted analog datastream to a digital symbol datastream; h) asecond filter cut-off frequency determining device for providing asecond filter cut-off frequency; i) a second low-pass filtering devicefor low-pass filtering of the digital symbol datastream on the basis ofa second filter cut-off frequency predetermined by the second filtercut-off frequency determining device, to provide an equalized symboldatastream; j) a decimation device for decimating the equalized symboldatastream in order to provide a decimated received symbol datastreamhaving discrete multitone symbols; and k) a multitone receiver devicefor further processing of the received symbol datastream.
 11. Thecircuit arrangement as claimed in claim 10, wherein the first low-passfiltering device is constructed as a first adaptive filtering device.12. The circuit arrangement as claimed in claim 10, wherein the secondlow-pass filtering device is a second adaptive filtering device.
 13. Thecircuit arrangement as claimed in claim 11, wherein the second low-passfiltering device is a second adaptive filtering device.
 14. The circuitarrangement as claimed in claim 10, wherein the first low-pass filteringdevice is constructed as a first filtering device which can bereprogrammed in the first filter cut-off frequency.
 15. The circuitarrangement as claimed in claim 11, wherein the first low-pass filteringdevice is constructed as a first filtering device which can bereprogrammed in the first filter cut-off frequency.
 16. The circuitarrangement as claimed in claim 12, wherein the first low-pass filteringdevice is constructed as a first filtering device which can bereprogrammed in the first filter cut-off frequency.
 17. The circuitarrangement as claimed in claim 11, wherein the second low-passfiltering device is constructed as a second filtering device which canbe reprogrammed in the second filter cut-off frequency.
 18. The circuitarrangement as claimed in claim 12, wherein the second low-passfiltering device is constructed as a second filtering device which canbe reprogrammed in the second filter cut-off frequency.
 19. The circuitarrangement as claimed in claim 13, wherein the second low-passfiltering device is constructed as a second filtering device which canbe reprogrammed in the second filter cut-off frequency.